How strange can space-time get?

Our own planet twists the fabric of space-time, as shown in this animation from "The Fabric of the Cosmos."

Theoretical physicist Brian Greene admits that the world he describes in his new public-TV documentary series, "The Fabric of the Cosmos," is nothing like everyday experience. He's not even sure some of the things he describes are for real. For example, how can we possibly know other universes exist? Believe it or not, there are ways to find out.

The four-part "Nova" series makes its debut on PBS stations on Wednesday night with an episode that delves into the mysteriously substantial properties of empty space. "As it turns out, empty space is not nothing," the Columbia University professor says at the start of the show. "It's something. ... So real, that empty space itself helps shape everything in the world around us, and forms the very fabric of the cosmos."

That episode is already available for watching over iPhones, iPads and iPod Touch devices, as well as through Amazon Prime instant video. And if you miss seeing it on TV on Wednesday, you'll be able to catch up with it later online. Over succeeding weeks, Greene addresses not only space, but also the nature of time, the weird world of quantum mechanics and the possibility that our universe is just one bubble in the cosmic ocean (or raisin-bread loaf, or cheese wedge) of the multiverse.

Most of the substance in "The Fabric of the Cosmos" comes from Greene's book of the same name — but the part about the multiverse is more speculative, and is derived from Greene's follow-up book titled "The Hidden Reality." So of course that's where I had to start when I had a chat with Greene this week. Here's an edited transcript of the Q&A:

Cosmic Log: You must get this question all the time: What sort of proof do you have that any of this stuff is true?

Brian Greene: Well, the first three episodes — focusing on space, time and quantum mechanics — are much more closely tied to observations and experiments that have already been done. Much of what we describe in those programs is firmly rooted in science that is now largely accepted, even though it's weird. The fourth program is different in that regard, because as the last program in the series, it is looking beyond what we currently know, and surveying the landscape of possibilities that may in the future become accepted science. But not yet.

That's the multiverse. The multiverse is hard to test because we have access to this universe, and the theory proposes that there are other universes. We can't directly see them. We can't visit them. So how would you ever prove that idea?

In the program, we tackle that issue head-on. We describe how the multiverse naturally emerged from investigations that were rooted in observations and experiments: things to do with questions of the origin of the universe, the cosmic microwave background radiation, issues surrounding the puzzles of the big bang that can only be resolved through an inflationary view of the universe — which then yields the multiverse. But it won't be a satisfying explanation until we have some kind of direct confirmation.

To my mind, there's really one main way that could happen in the near future: In this proposal, different universes are like different expanding bubbles in some larger cosmic environment, like bubbles in a bubble bath. And when bubbles in a bubble bath expand, they can run into each other. Similarly, expanding universes can collide. The math indicates that if and when they do, the collision can send ripple-like disturbances through the microwave background radiation — the heat signature left over from the big bang. Those temperature differences of that particular sort are something that people are looking for. Some even claim they've seen the first tentative signals of the pattern. I'm highly skeptical about that, as everybody should be. But this could be a way to subject an idea that seems so foreign to an observational test.

Q: There's also been some talk about possible observations at the Large Hadron Collider that might suggest energy was "leaking" into other dimensions.

A: Yes, the notion that there are extra dimensions provides another way in which you could have other universes. Our universe might be one piece of bread in a big cosmic loaf, where the other "slices" are displaced from ours in a new direction, and are actually other universes. One way to check that idea would be to have a very energetic collision of particles in our universe, on our slice of space. The math shows it's possible for energy from those collisions to be ejected off our slice and migrate into the wider cosmos. We would notice that here by seeing that energy was not conserved. The energy after the collision would be a little less than it was before, because some of the energy would have crossed beyond our universe.

The point is that there are strange ideas about the universe that can nevertheless yield evidence, if we know where to look.

WGBH

In "The Fabric of the Cosmos," physicist Brian Greene graphically shows how the "Mona Lisa" ... and even Brian Greene ... could exist in more than one universe.

Q: One of the points of the series is that there's a deeper level to reality that what we see in everyday life, suggested by mathematics and physics. You use all sorts of animations and graphics to convey a sense of the underlying fabric of the cosmos. Do you have some favorite tricks that you've used in the TV series?

A: When you're dealing with subjects that are abstract ... these are mind-bending ideas, but what do you point the camera at? That's a funny thing, because everything we do takes place within space, within time. The concepts of space and time are so profoundly interwoven with reality as we know it, and yet science has revealed that there are features of space and time that run completely counter to our intuition — if you examine them on non-human scales, that is, scales that are very tiny, or very big, or when you're moving very fast, or if you're near a very strong, massive gravitational object.

Since we can't actually go to those exotic realms, we use animation to show what it would be like if you could shrink down to a billionth of a billionth of a meter ... or what it'd be like if you could travel at just a tiny fraction less than the speed of light ... or what it would be like to hover near the edge of a black hole and then come back to Earth. And we use animation to show the largest bird's-eye view of the cosmos if some of these multiverse ideas are correct. That really gives you a visceral understanding of the concepts.

Q: In fact Einstein used these types of thought experiments as well when he worked on his theories of relativity. He imagined what it would be like to ride a light wave, or to be falling through space in an elevator...

A: If only Einstein had the tools of animation, who knows how far he would have gone!

But there's a serious point here: When I do my own work, I'm constantly trying to build a mental image of what's going on. I'm never comfortable if my understanding is just completely in the equations. I feel like I have a storehouse of imagery built up just from the scientific research itself, which then leads to a form that will work in a book or on TV, which requires dressing it up in various ways. The whole idea of trying to visualize abstract equations is something that many of us do as part of our second nature, as researchers.

Q: Are there any favorite visualizations you keep coming back to?

A: Well, sure. A lot of my work has to do with extra dimensions of space. And I readily admit that I cannot picture anything more than three dimensions. So in my own work, I'm constantly doing what we do in the television program, which is to use lower dimensions as analogies — two-dimensional analogies that you can draw and manipulate. You use those as a guide to what's happening in higher-dimensional settings, where the equations of string theory reside.

You have to be careful. Sometimes a lower-dimensional analogy can be misleading. But you begin to build up the art of knowing what aspects of those visualizations you can trust when you're taking the leap to higher dimensions, and which aspects make you say, "No, no, no, that won't give me insight into my real interest."

Q: What do you hope viewers will take away from the show?

A: The main goal is for people to leave the program with a more complete sense that when it comes to the universe, what you see is not what you get. There are layers upon layers of reality that we are unaware of in everyday life. Intuition is built up from experience, and our experiences since we appeared on the planet has been largely dictated by what is beneficial for our survival. Understanding the quantum world, and understanding the possibility of other universes, and understanding the deep nature of time don't help you get the next meal. So there hasn't been any real evolutionary pressure for us to gain intuition about those things. But when we have the luxury of thinking about them mathematically, we learn that there's much more to the universe than meets the eye.

It's absolutely thrilling to learn that time for me is not the same as time for you; that out there in space, time is elapsing at a different rate near the edge of a black hole; that in the depths of space, there is unavoidable, ferocious quantum activity; that the world is governed by probabilities, not certainty; and then there's entanglement, the idea that what you do over here can have a direct effect on something over there. Wow!

What is space? "The Fabric of the Cosmos" explains that empty space is not really empty.

Wow indeed. The TV show is just the tip of the iceberg: The "Fabric of the Cosmos" website offers tons of videos, interactives, intereviews and links to online resources. More than a dozen "Cosmic Cafes" have been organized nationwide to talk about space, time and the multiverse. And the World Science Festival has organized a screening of the first episode at Columbia University at 9 p.m. ET Wednesday, to be followed by a forum featuring Greene, theoretical physicist Leonard Susskind and Nobel laureate Saul Perlmutter.

Even though the in-person event sold out almost immediately, you can still tune in to live streaming video and join the discussion via Facebook or Twitter. I have an alternate suggestion: Watch the episode in advance, or save it for later, and tune in to "Virtually Speaking Science" at 9 p.m. ET Wednesday for my chat with interstellar-travel expert Marc Millis. Then, at 10 p.m. ET, switch on over to the World Science Festival's forum.